The invention relates to a self-penetrating fastening system including a rivet, a punch and a pressure pad (die), the rivet consisting of a head and shank with central cavity whose free end surface does not completely penetrate a workpiece (metal plate) receiving it.
A self-penetrating fastening system such as this is described in German Patent DE 39 42 482 C1. In the embodiment explained, two plates of different thicknesses, parts of the wall of a container, are connected to each other, particular value being attached to water and gas tightness. Corrosion stability is also taken into account in this state of the art.
If such self-penetrating fastening systems are also to be used in the area of automotive technology, for example in order to connect body sheets together, other essential factors are involved in addition to the corrosion stability in question. Because of the high mechanical stresses occurring, the strength of the connection of two plates is of decisive importance. In addition, depending on the place where the connection is applied (uncovered body areas), it should be possible to make the top of the rivet head as nearly flush as possible with the surrounding plate surface after manufacture. The process of producing the riveted connection should harm the surface areas in question as little as possible (e.g., through deformation resulting from warping of the sheet), so as to hold minimum any finishing operations required to improve the appearance.
The state-of-the-art riveted connection does not meet these additional requirements to the extent desired.
Hence it is the object of the invention to develop the state-of-the-art self-penetrating fastening device in such a way that the riveted connection produced by the device can meet high requirements with respect to its strength and so that the surface of the workpiece will not be permanently impaired.
It is claimed for the invention that this is accomplished in that the self-penetrating fastening system possesses the additional features presented in the descriptive portion of claim 1.
As a result of the design of the shank cavity in the area of the free end surface in the area of the free end surface, the cavity being conical in the broadest sense of the word, as the rivet penetrates a workpiece more deeply the resulting higher expansion force causes the radial movement of the rivet shank outward (effects expansion) to reach an order of magnitude such that an expansion angle  greater than 45xc2x0 may be reached. On the other hand, this process results in an undercut increasing the strength, the direct tensile strength in particular (end area of the rivet shank projecting radially further than the convexity of the lower sheet adjoining the jacket of the free end surface and the more or less rounded transition of the shaft into the rivet head during riveting also result in the gentlest possible treatment of the upper sheet during penetration of an upper sheet as illustrated in the embodiment, and also make certain that, among other things, if the rivet head has a flat top, this top can be embedded so as to be flush with the surrounding surface of the sheet and so that the circular notch marking the transition of rivet head to sheet surface is very narrow and extremely shallow.
Supplementary developments of the invention are described in the dependent claims. In addition to the preferred embodiment indicated, it is to be regarded as a special development (claim 6) that the circumferential wall of the die cavity is designed so as to be radially movable. Especially in the case of thin sheets this involves a supporting effect during cutting of the top sheet, without the process of spreading of the rivet shaft in the bottom plate being impaired in any way whatever, since the spring-mounted groove blocks (claim 7) can move radially outward as required.